= Normal-incidence grating width bound
{title2=$W\gtrsim\lambda^2/(\delta\lambda|\sin\theta|)$}
At normal incidence, an ideal grating of $N$ lines and period $d$ resolves a small <wavelength> separation $\delta\lambda$ in order $m$ by the Rayleigh condition $|m|N\gtrsim\lambda/\delta\lambda$. Combining this with $|m|\lambda=d|\sin\theta|$ yields $W=Nd\gtrsim\lambda^2/(\delta\lambda|\sin\theta|)\ge\lambda^2/\delta\lambda$. The final expression is a limiting lower bound as diffraction approaches grazing. Finite usable diffraction angle, unequal line strengths, aberrations and intrinsic linewidth can all demand a larger instrument or a more detailed criterion.
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